This invention relates to exhaust gas cleaner systems including catalytic converters for cleaning the exhaust gas of the internal combustion engines, and more particularly to the secondary air introduction systems for introducing secondary air into the exhaust pipe of the engine for enhancing the cleaning efficiency of the catalytic converters.
Catalytic converters are provided on the exhaust pipe of the automotive internal combustion engines for cleaning the exhaust gas. When heated above the reaction temperature, the catalytic converters remove the noxious components efficiently from the exhaust gas. Immediately after the start of the engine, however, the temperature of the catalytic converter is still low and hence the cleaning efficiency thereof is insufficient. To enhance the cleaning efficiency, fresh air (secondary air) is introduced into the exhaust pipe upstream of the catalytic converter. The air promotes the oxidation of noxious components such as HC (hydrogen carbide) and CO (carbon monoxide), and thereby raises the temperature of the catalytic converter quickly and enhances the cleaning efficiency (the efficiency to remove the noxious components) thereof.
FIG. 1 is a block diagram showing an internal combustion engine provided with a catalytic converter supplied with secondary air through an air introduction pipe. To an automotive internal combustion engine 1 are connected an air intake pipe 2 for supplying air to the engine and an exhaust pipe 3 for exhausting the exhaust gas to the atmosphere. The exhaust gas contains noxious components generated by the combustion within the cylinders of the internal combustion engine 1. The noxious components include nitrogen oxides (NO.sub.x) which are produced in abundance at higher temperatures and should be reduced by the catalytic converter for removal, and HC (hydrogen carbide) and CO (carbon monoxide), which are produced in abundance at lower temperatures and should be oxidized by the catalytic converter for removal.
At an upstream point of the air intake pipe 2 is disposed an air cleaner 4 for removing dusts contained in the air supplied to the cylinders of the engine. To the downstream side of the air cleaner 4 is mounted an airflow sensor 5 for measuring the flow rate of air taken into the cylinders of the internal combustion engine 1. Further downstream of the airflow sensor 5 is disposed a throttle valve 6 for adjusting the amount of airflow supplied to the cylinders of the engine.
At a middle point of the exhaust pipe 3 is disposed a catalytic converter unit 7 accommodating the catalytic converter (e.g., catalytic converter rhodium) for cleaning the exhaust gas by means of chemical reactions. An air introduction pipe 8 is coupled across a point of the air intake pipe 2 downstream of the air cleaner 4 and a point of the exhaust pipe 3 upstream of the catalytic converter unit 7. An air pump 9 forces the air from the air intake pipe 2 into the exhaust pipe 3, and introduces the secondary air into the exhaust pipe 3. A check-valve 10 prevents the exhaust gas from reversing through the air introduction pipe 8 into the air intake pipe 2.
An air/fuel ratio sensor 11 on the exhaust pipe 3 detects the oxygen concentration of the exhaust gas. Fuel injectors 12 are provided for respective cylinders of the engine in the respective branches of the air intake pipe 2. From the output of a crank angle sensor 13 is detected the rpm of the engine. The outputs of the airflow sensor 5, the air/fuel ratio sensor 11, and the crank angle sensor 13 are supplied to an engine controller 14, which controls the operations of the fuel injectors 12 in response to the outputs of the sensors 5, 11 and 13. The ignition plugs (not shown) for the respective cylinders are supplied from an ignition coil 15. The current supply to the ignition coil 15 is controlled by an igniter 16, which is controlled by the engine controller 14.
The operation of the secondary air introduction system for the catalytic converter 7 of FIG. 1 is as follows. A part of the air passing through the air cleaner 4 is sucked into the air introduction pipe 8 by means of the air pump 9 and introduced, through the check-valve 10, into the exhaust pipe 3 at a point upstream of the catalytic converter unit 7. The secondary air thus introduced into the exhaust pipe 3 is mixed with the exhaust gas exhausted from the cylinders of the engine and then is passed together through the catalytic converter unit 7. The secondary air supplied through the air introduction pipe 8 promotes the oxidation of the noxious components such as HC and CO into corresponding innocuous compounds such as H.sub.2 O (water) and CO.sub.2 (carbon dioxide). The exhaust gas passed through the catalytic converter is released into the atmosphere.
The fuel injectors 12 are controlled by the engine controller 14. The amount of the injected fuel is determined primarily on the basis of the output of the airflow sensor 5 and the rpm of the engine calculated from the output of the crank angle sensor 13. The amount of the injected fuel thus determined is corrected on the basis of the output of the air/fuel ratio sensor 11, etc., and the fuel injectors 12 are driven in accordance with the corrected amount.
The igniter 16 controls the ignition timings of the respective cylinders by turning on and off the current supplied to the ignition coil 15. The engine controller 14 controls the operation of the igniter 16 in response to the output of the airflow sensor 5 and the rpm of the engine. The air pump 9 is driven from the time when the engine is started to the time when it is stopped, and keeps on introducing the secondary air into the exhaust pipe 3.
As discussed above, the fresh air supplied through the secondary air introduction system of FIG. 1 promotes the oxidation of noxious component such as HC and CO and thereby quickens the temperature rise of the catalytic converter. Thus, the time needed for the activation of the catalyst is shortened (compared with the case where no secondary air introduction system is provided) and the cleaning efficiency of the catalytic converter rises relatively quickly.
However, the legal control against the noxious components contained in the exhaust gas is becoming increasingly strict. The exhaust gas regulation of California is an example. Thus further reduction of the noxious components in the exhaust gas is an urgent need.
FIG. 2 is a block diagram showing an internal combustion engine provided with two catalytic converter units supplied with secondary air through an air introduction pipe. The arrangement of FIG. 2 is disclosed, for example, in Japanese Laid-Open Utility Model Application (Kokai) No. 47-21018.
The exhaust gas cleaner system of FIG. 2 includes first and second catalytic converter units 7a and 7b. The second catalytic converter unit 7b is disposed downstream of the first catalytic converter unit 7a. The first and second catalytic converter units 7a and 7b accommodate the catalytic converter rhodium, respectively. The air introduction pipe 8 includes two branches for supplying separate amounts of secondary air to the first and second catalytic converter units 7a and 7b. The amounts of air supplied to the first and second catalytic converter units 7a and 7b are adjusted by a change-over valve 10a. In response to the output of a temperature sensor 21 disposed at the second catalytic converter unit 7b, a controller 22 controls the change-over valve 10a and thereby adjusts the amounts of the secondary air supplied to the first and second catalytic converter units 7a and 7b. The parts not shown in FIG. 2 are similar to those shown in FIG. 1.
The operation of the secondary air introduction system for the first and second catalytic converter units 7a and 7b of FIG. 2 is as follows. When the choke valve is operated to restrict the airflow through the air intake pipe 2 at the start of the engine, the air/fuel mixture supplied to the cylinders of the engine is rich in fuel content and the exhaust gas contains much HC and CO. Under this circumstance, the air sucked in by the air pump 9 from the air intake pipe 2 into the air introduction pipe 8 is divided by the change-over valve 10a into two portion supplied through the two branches to the first and second catalytic converter units 7a and 7b, respectively. The noxious components such as HC and CO are thus oxidized into innocuous components such as H.sub.2 O and CO.sub.2 both in the first and second catalytic converter units 7a and 7b, and are removed from the exhaust gas.
When the temperature of the first and second catalytic converters 7a and 7b rises and the catalysts are activated, the high temperature of the second catalytic converter unit 7b is detected by the temperature sensor 21. In response to the output of the temperature sensor 21, the controller 22 changes over the change-over valve 10a to turn off the supply of secondary air to the first catalytic converter unit 7a. The secondary air is thus supplied exclusively to the the second catalytic converter unit 7b. The first catalytic converter 7a thus begins to efficiently reduce the nitrogen oxides NO.sub.x contained in the exhaust gas to the nitrogen gas N.sub.2. The remaining noxious components such as HC and CO are oxidized into innocuous components such as H.sub.2 O and CO.sub.2 exclusively in the second catalytic converter unit 7b.
Even the exhaust gas cleaning system of FIG. 2 is not sufficiently effective. Namely, at the initial time when the first and the second catalytic converters are still at a lower temperature, the air lower in temperature than the exhaust gas are introduced into the exhaust pipe 3 and mixed with the exhaust gas. The temperature of the exhaust gas is thus reduced, thereby slowing down the temperature rise of the catalytic converters and delaying the full activation thereof. The initial cleaning efficiency is thus reduced. Furthermore, the air is distributed initially in a fixed ratio to the first and the second catalytic converters. The NO.sub.x, however, becomes increasingly abundant as the temperature of the internal combustion engine 1 rises. Until the change-over valve 10a is switched to turn off the air supply to the first catalytic converter unit 7a, this increasing amount of NO.sub.x are not removed efficiently.
FIG. 3 is a block diagram showing an internal combustion engine provided with a catalytic converter supplied with heated secondary air through an air introduction pipe. In FIG. 3, the internal combustion engine 1 is shown with a transmission 1a and an AC generator 28. As in the case of FIGS. 1 and 2, the air taken in through the air cleaner 4 is supplied to the cylinders of the internal combustion engine 1 through the air intake pipe 2. The amount of airflow is controlled by the throttle valve 6 in the air intake pipe 2. The exhaust gas is released to the atmosphere through the exhaust pipe 3 provided with a catalytic converter unit 7.
The air sucked in by the electric air pump 9a into the air introduction pipe 8a is introduced into the exhaust pipe 3 through a flow control valve 10b, a check-valve 10, and a heater 23. The flow control valve 10b controls the amount of the air introduced into the exhaust pipe 3 through the air introduction pipe 8a. The check-valve 10 prevents the exhaust gas from reversing through the air introduction pipe 8a. The heater 23 heats the air before introducing it into the exhaust pipe 3. The controller unit 24 controls the operation of the flow control valve 10b as well as the ON/OFF of the relays 26 and 27. The relays 26 and 27 control the supply of current from the battery 25 to the electric air pump 9a and the heater 23, respectively.
The operation of the secondary air introduction system for the catalytic converter unit 7 of FIG. 3 is as follows. As described above, immediately after the start of the engine, the air/fuel ratio is small and the air/fuel mixture is rich in fuel content. The exhaust gas thus contains large amounts of CO and HC. The catalytic converter is still below the reaction (activation) temperature and is not sufficiently activated yet.
Simultaneously with, or after a predetermined length of time after, the start of the engine, the controller unit 24 turns on the relay 26 for the electric air pump 9a, thereby supplying power from the battery 25 to the electric air pump 9a. The electric air pump 9a is thus driven and introduces the secondary air into the exhaust pipe 3 through the air introduction pipe 8a. Further, simultaneouly with, or after a predetermined length of time after, the start of the engine, the controller unit 24 turns on the relay 27 for the heater 23, thereby supplying power from the battery 25 to the heater 23. The heater 23 thus heats the secondary air before it is introduced into the exhaust pipe 3.
The heated secondary air introduced into the exhaust pipe 3 is supplied to the catalytic converter unit 7 together with the exhaust gas, and accelerates the temperature rise of the catalytic converter. The catalytic converter thus quickly reaches the reaction temperature and is activated. The noxious components such as CO and HC are converted into the innocuous components such as CO.sub.2 and H.sub.2 O in the catalytic converter unit 7. The exhaust gas passed through and cleaned by the catalytic converter is released to the atmosphere.
During the above operation, the electric air pump 9a and the heater 23 are operated simultaneously. The amount of consumed current is thus large, and the AC generator 28 keeps on charging the battery 25. As shown in FIG. 4, the amount of secondary air introduced into the exhaust pipe 3 remains substantially constant. The amount of secondary air is controlled by the flow control valve 10b in the air introduction pipe 8a.
The secondary air introduction system for the catalytic converter unit 7 of FIG. 3 has the following disadvantage. During operation, the electric air pump 9a and the heater 23 is supplied with current from the battery 25, which should thus be charged continually by the AC generator 28. As a result, the AC generator 28 constitutes a heavy load on the internal combustion engine 1. When the output of the internal combustion engine 1 is reduced (as when the engine is idling), the concentration of the noxious components in the exhaust gas increases due to the heavy load imposed by the AC generator 28.